1. Field of the Invention
The present invention relates to a method for controlling a system for metering and mixing a product consisting of a first component and of a second component that have to be mixed in a mixer at a predetermined metering ratio.
Moreover, the present invention also relates to such a metering and mixing system in which the control method can be applied, and a spraying or extrusion device comprising such a metering and mixing system.
The role of this system is to deliver a mixture of at least two components in a ratio defined by an operator. This mixture is then used directly in the applications, for example for the spraying of a paint with the aid of a spraying device comprising a gun that is connected to this metering and mixing system.
In order to meter and mix a two-component product, it is known practice to use a system of this kind comprising a first double-acting, reciprocating, vertical pump capable of feeding the first component into the mixer, and a second double-acting, reciprocating, vertical pump capable of feeding the second component into the mixer, usually in sequence relative to the first pump. The movement of the piston of each pump can be reversed from a direction of expulsion of the component to a direction of suction and expulsion of the component and vice versa. The circuit of each pump comprises, in addition to a suction valve and a discharge valve associated with the pump, a feed valve inserted between the discharge valve of each pump and the mixer.
2. Description of the Related Art
A known control method comprises the following phases consisting in:                opening the feed valve of the first pump and feeding a predefined dose into the mixer as a function of the said predetermined dose ratio;        closing the feed valve of the first pump;        opening the feed valve of the second pump and feeding a predefined dose into the mixer as a function of the said predetermined dose ratio;        closing the feed valve of the second pump so as to terminate the phases of a first feed cycle;        triggering the reversal of the piston of each pump in its suction and expulsion direction when it is necessary to again fill this pump with the corresponding component; and        reopening the feed valve of the first pump in order to begin the next cycle by repeating these phases.        
In this known system with sequential feeding, the reversal of the movement of the piston of the pump from the expulsion direction to the suction and expulsion direction of the pump and vice versa is triggered by an end-of-travel abutment of the piston of the pump.
U.S. Pat. No. 6,896,152 describes another sequential-feeding system. According to this document, a device for delivering a material with several components under pressure by using a spray gun comprises two position sensors of which each is associated with a respective drive/pump device assembly for delivering increments of resolution corresponding to a fixed volume fed by the pump into a mixer. A controller is connected to the position sensors in order to reciprocatingly move one of the drive/pump device assemblies by one increment. The other drive/pump device is then moved by an increment in order to adjust the predetermined metering ratio.
The drawback of these known systems is that they do not allow the pumps to be used optimally, in particular because this operation causes an interruption in the flow rate when there is a reversal of one of the pumps before being able to continue this sequential feeding.
In its patent application FR2928466, the applicant proposed a solution for alleviating the drawbacks of the known metering and mixing systems. According to this patent application, the reversal of the first pump is carried out at the same time as the second pump feeds a dose of its component, and vice versa. Consequently, the reversal of the first pump takes place concurrently.
Document U.S. Pat. No. 4,440,314 describes a method for controlling a system for metering and mixing a product consisting of at least two components designed to be mixed in a mixer, at a predetermined metering ratio, the system comprising a reciprocating pump for each of the components, the feeding of the first component being continuous and the feeding of the second component being continuous and proportional to that of the first component in order to maintain the metering ratio.